From the viewpoint of global environment protection, automobile lightening has strongly been desired for the purpose of making fuel-efficient automobiles. When a steel sheet is used for parts composing a vehicle, lightening has been attempted by applying a high-strength steel sheet and reducing the sheet thickness of this steel sheet. On the other hand, to improve the collision safety of automobiles, further strengthening has been required for automobile parts, such as pillars, and there has been an increasing need for ultrahigh-strength steel sheets having higher tensile strength.
However, when thin steel sheets are made to have higher strength, the elongation EL or r value (Lankford value) thereof is lowered, resulting in the deterioration of press formability or shape fixability.
Under these circumstances, to realize high-strength structural parts for automobiles, a hot pressing method (a so-called “hot press method”) has been proposed (e.g., Patent Document 1), in which both press-forming and improving the strength of parts by hardening are achieved at the same time. This technique is a method in which a steel sheet is heated up to an austenite (γ) region not lower than an Ac3 transformation point thereof and then hot press-formed, during which the steel sheet is simultaneously hardened by being brought into contact with a press tool at ordinary temperature, to realize ultrahigh strengthening.
According to such a hot pressing method, the steel sheet is formed in a state of low strength, and therefore, the steel sheet exhibits decreased springback (favorable shape fixability), resulting in the achievement of a tensile strength in the 1500 MPa class by rapid cooling. In this regard, such a hot pressing method has been called with various names, in addition to a hot press method, such as a hot forming method, a hot stamping method, a hot stamp method, and a die quenching method.
FIG. 1 is a schematic explanatory view showing the structure of a press tool for carrying out hot press-forming as described above (hereinafter represented sometimes by “hot pressing”). In FIG. 1, reference numerals 1, 2, 3, and 4 represent a punch, a die, a blank holder, and a steel sheet (blank), respectively, and abbreviations BHF, rp, rd, and CL represent a blank holding force, a punch shoulder radius, a die shoulder radius, and a clearance between the punch and the die, respectively. In these parts, punch 1 and die 2 have passage 1a and passage 2a, respectively, formed in the inside thereof, through which passages a cooling medium (e.g., water) can be allowed to pass, and the press tool is made to have such a structure that these members can be cooled by allowing the cooling medium to pass through these passages.
When a steel sheet is hot pressed (e.g., subjected to hot deep drawing) with such a press tool, the forming is started in a state where a blank (steel sheet 4) is softened by heating to a temperature not lower than an Ac3 transformation point thereof. That is, steel sheet 4 is pushed into a cavity of die 2 (between the parts indicated by reference numerals 2 and 2 in FIG. 1) by punch 1 with steel sheet 4 in a high-temperature state being sandwiched between die 2 and blank holder 3 to form steel sheet 4 into a shape corresponding to the outer shape of punch 1 while reducing the outer diameter of steel sheet 4. In addition, heat is removed from steel sheet 4 to the press tool (punch 1 and die 2) by cooling punch 1 and die 2 in parallel with the forming, and the hardening of a material is carried out by further retaining and cooling steel sheet 4 at the lower dead point in the forming (the point of time when the punch head is positioned at the highest level: the state shown in FIG. 1). Formed products with high dimension accuracy and strength in the 1500 MPa class can be obtained by carrying out such a forming method. Furthermore, such a forming method results in that the volume of a pressing machine can be made smaller because a forming load can be reduced as compared with the case where parts in the same strength class are formed by cold pressing.